Thermal mass is a concept in building design which describes how the mass of the building provides “inertia” against temperature fluctuations, sometimes known as the thermal flywheel effect. For example, when outside temperatures are fluctuating throughout the day, a large thermal mass within the insulated portion of a house can serve to “flatten out” or dampen the daily temperature fluctuations, since the thermal mass absorbs heat when the surroundings are hotter than the mass, and give heat back when the surroundings are cooler. This is distinct from a material’s insulative value, which reduces a building’s thermal conductivity, allowing it to be heated or cooled relatively separate from the outside, or even just retain the occupants body heat longer.

Thermal mass is particularly beneficial where there is a big difference between day and night outdoor temperatures especially in hot dry climate. The day/night changes in temperature and solar radiation pose challenges for maintaining human thermal comfort in buildings. Passive and energy-conserving buildings seek to manage the available thermal energy by lowering peaks and dampening the fluctuations in order to maintain conditions for human comfort. When used judiciously, thermal mass can delay heat flow through the building envelope by as much as 10 to 12 hours producing a warmer house at night in winter and a cooler house during the day in summer. When combined with passive solar design, thermal mass can play an important role in providing major reductions to energy use in active heating and cooling systems. Massive building envelopes-such as masonry, concrete, earth, and insulating concrete forms (ICFs) can be utilized as one of the simplest ways of reducing building heating and cooling loads. This article analyses the role and effectiveness of thermal mass as a strategy for providing indoor thermal comfort and energy conservation in buildings.

Heat Transferred Through Roof and Walls

The average value of the rate of heat transferred through an opaque building element can be written as:
Qopaq = Aopaq Uopaq (Tso – TRo)
where,
Tso = Average Sol-air Temperature or effective temperature of the opaque surface (oC),
TRo = Average Room Temperature (oC),
Uopaq = Overall Conductivity of Opaque surface (W/m2 oC), and
Aopaq = Surface Area of opaque component (m2).

When building elements are subjected to temperature fluctuations, not only the heat transmitted by the element but also the time taken for heat to travel through the element becomes important. A fluctuating temperature wave outside the building element, gives rise to another temperature wave with lower amplitude on the inside. The two temperature waves are out of phase by an angle called time lag, which depends upon the thermal ‘diffusivity’, and thickness of the building element. In Figure 1 time lag is given by the difference in hours between the occurrence of the peak temperature outdoors and the corresponding peak temperature indoors. The ratio of the amplitude of internal temperature wave to the amplitude of external temperature wave is called the “decrement factor”. Massive building elements such as brick walls usually have a large time lag and lower decrement factor than thinner elements of lightweight materials.

Effect of Thermal Mass on Indoor Temperature

The ability to store energy in a thermal mass is a vital strategy in controlling and ameliorating the building microclimate and lowering energy demand heavily weighing on the infrastructure. Rising living standards on the one hand, and the environmental implications of increased fossil fuel exploitation on the other hand, indicate the need for alternative solutions, and a more sane use of energy in buildings. High thermal capacity in a shaded and insulated building can help lower indoor maxima by 35-45% of the outdoor ones when the building is unventilated (Givoni, 1994)

In a study done by Shaviv, an analysis for the determination of the reduction in the maximum indoor temperature compared with the maximum outside temperature (Tmax) was carried out using an hourly simulation model ENERGY to predict the thermal performance of the building. The results obtained show that in the hot humid climate, it is possible to achieve a reduction of 3–6°C in a heavy constructed building without operating an air conditioning unit. The exact reduction achieved depends on the amount of thermal mass, the rate of night ventilation, and the temperature swing of the site between day and night (Shaviv et al, 2001).

Hasan Fathy conducted tests on experimental buildings located at Cairo Building Research Centre, using different materials. The materials used were mud brick walls and roof 50 cm thick and prefabricated concrete panel walls and roof 10 cm thickness. The thermal performance of the two buildings over a 24 hour cycle was monitored. The air temperature fluctuation inside the mud brick model did not exceed 2°C during the 24 h period, varying from 21-23°C which is within the comfort zone. On the other hand, the maximum air temperature inside the prefabricated model reached 36°C, or 13°C higher than the mud brick model and 9°C higher than outdoor air temperature. The indoor temperature of the prefabricated concrete room is higher than the thermal comfort level most of the day (Fathy, 1986). Moore reported the temperatures in and around an adobe building. It indicated that when the average inside and outside temperatures are about equal, the maximum interior temperature occurred at about 22:00 h (about 8 h after the outside peak). Furthermore, the outside temperature swing was about 24°C while the interior swing was about 6°C (Moore, 1993). The effect of thermal mass on interior temperature is shown in Figure 2.

Material Selection for Ideal Thermal Mass

In order to be effective as a thermal mass, a material must have a high heat capacity, a moderate conductance, a moderate density, and a high emissivity. It is also important that the material serve a functional (structural or decorative) purpose in the building. Among common building materials, wood does not make a good thermal mass because it not only has a low heat storage potential, but is also not very conductive. Rammed earth provides excellent thermal mass because of its high density, and the high specific heat capacity of the soil used in its construction.

Concrete and other masonry products are ideal, having a high capacity for heat storage, moderate conductance that allows heat to be transferred deep into the material for storage, high emissivity to allow absorption of more radiation than that which is reflected. When sized properly, concrete is effective in managing diurnal energy flow. Conveniently, structural concrete and thermal mass share common dimensions, so there is no wasted mass when building a structure. Insulating concrete forms are commonly used to provide thermal mass to building structures. Insulating Concrete Forms or ICF provide the specific heat capacity and mass of concrete. Thermal Inertia of the structure is very high because the mass is insulated on both sides. Water is also effective as a thermal mass in that it has high potential for heat storage and it can be effective in a diurnal thermal management scheme.

Steel, while having a seemingly high potential for heat storage, has two drawbacks—its low emissivity indicates that a large majority of the incident radiation is reflected, rather than absorbed and stored, and its high conductivity signals an ability to quickly transfer heat stored in the material’s core to the surface for release to the environment, thus shortening the storage cycle to minutes rather than the hours needed for diurnal thermal tempering. Glass also seems to have a high potential for heat storage, but it is relatively transparent to near infrared radiation and reflective of far infrared radiation. Adding pigments to glass (especially blue and green) increases its ability to absorb radiation, which can become a thermal problem during the cooling season.

Effective Placement of Thermal Mass

The cardinal rule for effective thermal mass is to place it inside the insulated skin of the building. A masonry wall with exterior insulation is a thermally elegant construction. It is effective during both heating and cooling seasons. During heating seasons it can store solar and internal gains by day and release the heat by night. During the cooling season it gains heat from interior sources during occupied periods and can be flushed of heat at night. A masonry wall that is insulated on the interior is thermally isolated from internal gains and solar gains that enter the building through glazing. Its mass is virtually useless in management of indoor temperatures. Uninsulated masonry walls are thermally perverse, conducting summer heat gains into the building (especially true of east- and west-facing walls) and conducting internal heat to the exterior in winter—granted, with a thermal lag determined by the thickness of the masonry. In vernacular architecture for hot arid climates this perversity is mitigated by using extremely thick (2–3 feet) masonry walls to avoid diurnal transmission of heat.

The Earth as a Thermal Mass

Building underground or berming a structure uses the mass of the earth as a means of moderating outside temperature extremes. In an earth sheltered buildings or earth bermed structures, the reduced infiltration of outside air and the additional thermal resistance of the surrounding earth considerably reduces the average thermal load. During the heating season, the earth cover effectively lowers the number of heating degree days by providing a comparatively high outdoor design temperature (equal to the earth temperature, rather than the air temperature). The cold air is separated from the building’s envelope by the earth and infiltration is practically eliminated. During the cooling season, the earth reduces the number of cooling degree days by preventing solar radiation from striking the building’s envelope.

Application of Thermal Mass in Different Climates

The correct use and application of thermal mass is dependent on the prevailing climate of a district or a region.

Temperate and cold temperate climates

Thermal mass is ideally placed within the building and situated where it still can be exposed to winter sunlight (via windows) but insulated from heat loss. The thermal mass is warmed passively by the sun or additionally by internal heating systems during the day. Heat stored in the mass is then released back into the interior during the night. It is essential that it be used in conjunction with the standard principles of passive solar design.

Any form of thermal mass can be used. A concrete slab foundation either left exposed or covered with conductive materials e.g. tiles; is one easy solution. Another novel method is to place the masonry facade of a timber-framed house on the inside (‘reverse-brick veneer’). Thermal mass in this situation is best applied over a large area rather than in large volumes or thicknesses. 7.5–10 cm (3-4”) is often adequate. Trombe wall, commonly constructed of concrete or masonry, are placed directly between the south-facing aperture and the interior space. The use of night insulation enhances the efficacy of this system considerably. When daytime comfort is a design goal, thermocirculation vents can provide warm air to raise the air temperature in the space.

Since the most important source of heat is from the sun, the ratio of glazing to thermal mass is an important factor to consider. Various formulas have been devised to determine this (Chiras, 2002). As a general rule, additional solar-exposed thermal mass needs to applied in a ratio from 6-8:1 for any area of north facing in Southern Hemisphere), south facing in Northern Hemisphere glazing above 7% of the total floor area. e.g. a 200 m2 house with 20 m2 of north facing glazing has 10% of glazing by total floor area; 6 m2 of that glazing will require additional thermal mass. Therefore, 36-48 m2 of solar-exposed thermal mass is required. The exact requirements vary from climate to climate.

Hot and arid climates (e.g. desert)

In hot and dry climates with a large diurnal range it is advantageous to use massive building elements. The effect of massive construction is to lower the maximum internal daytime temperature and to raise the minimum nighttime temperature while in lightweight construction; the internal temperatures follow closely the pattern of outdoor temperatures (Figure 2).

Masonry construction provides heat storage within the building structure due to its thermal capacity, which helps contain indoor temperature fluctuations and acts as interim heat sink. The importance of heat storage increases with larger swings in outdoor temperature. Heat dissipation is then achieved overnight by exposing the building structure to the cooler night-time outdoor air. The classical use of thermal mass includes adobe or rammed earth houses. Its function is highly dependent on marked diurnal temperature variations. The wall predominantly acts to retard heat flow from the exterior to the interior during the day. The high volumetric heat capacity and thickness prevents heat from reaching the inner surface. When temperatures fall at night, the walls re-radiate the heat back into the night sky. In this application it is important for such walls to be massive to prevent the ingress of heat into the interior.

One of the common occurrences of large parts of the Middle Eastern deserts are the hot spells. During summer these outdoor conditions may become unbearable in a matter of hours. This, in turn, may create indoor conditions that are even worse. Combined with dust storms, such hot spells may even prevent the use of outdoor spaces and the opening of windows to allow for comfort ventilation. Thermal mass provides probably the best – sometimes the only – solution, especially considering the possibility of power failures due to the storms or excess peak energy demand. Alternately, when such spells occur during the winter, thermal mass is able to take advantage of the high air temperatures and store heat (Meir, 2000).

Hot and humid climates (e.g. sub-tropical and tropical)

The use of thermal mass is the most challenging in this environment where night temperatures remain elevated. Its use is primarily as a temporary heat sink. However, it needs to be strategically located to prevent overheating. It should be placed in an area that is not directly exposed to solar gain and also allow adequate ventilation at night to carry away stored energy without increasing internal temperatures any further. If to be used at all it should be used in judicious amounts and again not in large thicknesses.

Conclusion

Thermal mass is an essential strategy for indoor climate control especially in hot dry climates. It has the potential to ameliorate indoor extremes as well as energy conservation in conditioned buildings. Thermal mass is one of the powerful tools which architects and designers can use to control temperature. It can be used to optimize the performance of energy-conserving buildings that rely primarily on mechanical heating and cooling strategies. In certain climates, massive building envelopes-such as masonry, concrete, earth, and insulating concrete forms (ICFs)-can be utilized as one of the simplest ways of reducing building heating and cooling loads. Such reductions in building envelope heat losses combined with optimized material configuration and the proper amount of thermal insulation in the building envelope help to reduce the building cooling and heating energy demands and building related CO2 emission into the atmosphere.